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Botanical name: Fucus vesiculosus


© Martin Wall

Parts used and where grown

Bladderwrack is a type of brown algae (seaweed) that grows on the northern Atlantic and Pacific coasts of the United States and on the northern Atlantic coast and Baltic coast of Europe. The main stem of bladderwrack, known as the thallus, is used medicinally. The thallus has tough, air-filled pods or bladders to help the algae float—thus the name bladderwrack. Although bladderwrack is sometimes called kelp, that name is not specific to this species and should be avoided.

Bladderwrack has been used in connection with the following conditions (refer to the individual health concern for complete information):

Science Ratings Health Concerns




Gastroesophageal Reflux Disease (GERD)




Iodine deficiency

Wound healing (topical)

3Stars Reliable and relatively consistent scientific data showing a substantial health benefit.
2Stars Contradictory, insufficient, or preliminary studies suggesting a health benefit or minimal health benefit.
1Star For an herb, supported by traditional use but minimal or no scientific evidence. For a supplement, little scientific support and/or minimal health benefit.

Historical or traditional use (may or may not be supported by scientific studies)

Bladderwrack’s mucilaginous thallus has long been used to soothe irritated and inflamed tissues in the body.1 It was also historically used as a bulk-forming laxative.2 People living near oceans or seas have a historically low rate of hypothyroidism, due, in part, to ingestion of iodine-rich food, such as seafood and seaweeds like bladderwrack. It has also been used to counter obesity, possibly due to its reputation for stimulating the thyroid gland. Clinical research in this area has failed to confirm that seaweeds like bladderwrack help with weight loss,3 though more specific research is warranted.

Active constituents

There are three major active constituents in bladderwrack: iodine, alginic acid, and fucoidan.

The amount of iodine in bladderwrack is highly variable,4 probably as a result of different amounts of iodine in the water where it grows. A reasonable portion of bladderwrack may contain the U.S. adult recommended dietary allowance (RDA) of iodine (150 mcg). The RDA amount of iodine is believed to be necessary for maintenance of normal thyroid function in adults (infants and children need proportionally less). Thus, in people with insufficient iodine in their diet, bladderwrack may serve as a supplemental source of iodine. Either hypothyroidism or goiter due to insufficient intake of iodine may possibly improve with bladderwrack supplementation, though human studies have not confirmed this.

Alginic acid is a type of dietary fiber that can be used to help relieve constipation and diarrhea. However, human studies have not been done on how effective bladderwrack is for either of these conditions. An over-the-counter antacid, Gaviscon®, containing magnesium carbonate and sodium alginate (the sodium salt of alginic acid), has been shown to effectively relieve the symptoms of heartburn compared to other antacids in a double-blind study.5 However, bladderwrack has not been studied for use in people with heartburn. Bladderwrack might also help indigestion, though again clinical trials have not been conducted. Calcium alginate (the calcium salt of alginic acid) has shown promise as an agent to speed wound healing in animal studies6 but has not been demonstrated to be effective in humans.

Alginic acid has also been shown to inhibit HIV in the test tube.7 However, this effect has not been studied in humans. Alginic acid may help lower LDL (“bad”) cholesterol levels, according to animal studies.8 No human trials have studied this effect of bladderwrack. It is widely used in food and pharmaceuticals as a thickener and gelling agent.9

Fucoidan is another type of dietary fiber in bladderwrack that contains numerous sulfur groups. According to test tube and animal studies, this appears to give fucoidan several properties, such as lowering LDL cholesterol levels,10 lowering blood glucose levels,11 anti-inflammatory activity,12 possible anticoagulant effects,13 and antibacterial14 and anti-HIV activity.15 Though it has not been definitively proven, fucoidan is thought to prevent bacteria and viruses from binding to human cells, a necessary step in starting an infection, as opposed to killing the microbes directly.16 17 To date, no human clinical trials have been done with fucoidan or bladderwrack to support their use for any of these conditions.

How much is usually taken?

For short-term use (a few days) to relieve constipation, powdered bladderwrack can be taken in the amount of 1 teaspoon three times per day along with at least 8 oz of water each time.18 For thyroid problems, gastritis, or heartburn, 5 to 10 grams of dried bladderwrack in capsules three times per day has been recommended. Alternately, bladderwrack may be eaten whole or made into a tea using 1 teaspoon per cup of hot water, allowing each cup to sit at least 10 minutes before drinking. Three cups per day of tea can be drunk. No more than 150 mcg iodine should be consumed from all sources, including bladderwrack, per day.19 However, most bladderwrack products do not give any indication of their iodine content. Therefore, anyone considering taking bladderwrack should first consult a physician trained in nutrition and herbal medicine.

Are there any side effects or interactions?

Bladderwrack is generally safe, though there are three potential problems with its consumption: acne, thyroid dysfunction, and heavy-metal contamination. Iodine in any form—including from bladderwrack and other seaweeds—can cause or aggravate acne in some people.20 Excessive iodine ingestion can cause either hypothyroidism or hyperthyroidism and should be avoided.21 22 Bladderwrack and other seaweeds that grow in heavy-metal-contaminated waters may contain high levels of these toxins (particularly arsenic and lead), leading to nerve damage,23 kidney damage,24 or other problems. Only bladderwrack known to have been harvested from clean water or labeled to indicate the absence of heavy metals or other contaminants should be consumed. The safety of using bladderwrack during pregnancy and breast-feeding is unknown. People who are allergic to iodine may need to avoid bladderwrack.

At the time of writing, there were no well-known drug interactions with bladderwrack.


1. Newall CA, Anderson LA, Phillipson JD. Herbal Medicines: A Guide for Health-Care Professionals. London: Pharmaceutical Press, 1996:124–6.

2. Mills SY. Out of the Earth: The Essential Book of Herbal Medicine. Middlesex, UK: Viking Arkana, 1991:514–6.

3. Björvell H, Rössner S. Long-term effects of commonly available weight reducing programmes in Sweden. Int J Obes 1986;11:67–71.

4. Norman JA, Pickford CJ, Sanders TW, et al. Human intake of arsenic and iodine from seaweed based food supplements and health foods available in the UK. Food Addit Contam 1987;5:103–9.

5. Chevrel B. A comparative crossover study on the treatment of heartburn and epigastric pain: Liquid Gaviscon and a magnesium-aluminum antacid gel. J Int Med Res 1980;8:300–3.

6. Barnett SA, Varley SJ. The effects of calcium alginate on wound healing. Ann R Coll Surgeons Engl 1987;69:153–5.

7. Béress A, Wassermann O, Bruhn T, et al. A new procedure for the isolation of anti-HIV compounds (polysaccharides and polyphenols) from the marine alga Fucus vesiculosus. J Nat Prod 1993;56:478–88.

8. Vázquez-Freire MJ, Lamela M, Calleja JM. Hypolipidaemic activity of a polysaccharide extract from Fucus vesiculosus L. Phytother Res 1996;10:647–50.

9. Lahaye M, Kaeffer B. Seaweed dietary fibres: Structure, physico-chemical and biological properties relevant to intestinal physiology. Sci Aliments 1997;17:564–84 [review].

10. Vázquez-Freire MJ, Lamela M, Calleja JM. Hypolipidaemic activity of a polysaccharide extract from Fucus vesiculosus L. Phytother Res 1996;10:647–50.

11. Vázquez-Freire MJ, Lamela M, Calleja JM. A preliminary study of hypoglycaemic activity of several polysaccharide extracts from brown algae: Fucus vesiculosus, Saccorhiza polyschides and Laminaria ochroleuca. Phytother Res 1996;10(suppl):S184–5.

12. Bartlett MR, Warren HS, Cowden WB, Parish DR. Effects of the anti-inflammatory compounds castanospermine, mannose-6-phosphate and fucoidan on allograft rejection and elicited peritoneal exudates. Immunol Cell Biol 1994;72:367–74.

13. Church FC, Mead JB, Treanor RE, Whinna HC. Antithrombin activity of fucoidan. The interaction of fucoidan with heparin cofactor II, antithrombin III and thrombin. J Biol Chem 1989;264:3618–23.

14. Criado MT, Ferreirós CM. Toxicity of an algal mucopolysaacharide for Escherichia coli and Neisseria meningitidis strains. Rev Esp Fisiol 1984;40:227–30.

15. Moen LK, Clark GF. A novel reverse transcriptase inhibitor from Fucus vesiculosus. Int Conf AIDS 1993;9(1):145, abstr. #PO-A03–0061.

16. Criado MT, Ferreirós CM. Toxicity of an algal mucopolysaacharide for Escherichia coli and Neisseria meningitidis strains. Rev Esp Fisiol 1984;40:227–30.

17. Lederman S, Gulick R, Chess L. Dextran sulfate and heparin interact with CD4 molecules to inhibit the binding of coat protein (gp120) of HIV. J Immunol 1989;143:1149–54.

18. Mills SY. Out of the Earth: The Essential Book of Herbal Medicine. Middlesex, UK: Viking Arkana, 1991:514–6.

19. Blumenthal M, Busse WR, Goldberg A, et al. (eds). The Complete German Commission E Monographs: Therapeutic Guide to Herbal Medicines. Austin: American Botanical Council and Boston: Integrative Medicine Communications, 1998:315.

20. Harrell BL, Rudolph AH. Kelp diet: A cause of acneiform eruption. Arch Dermatol 1976;112:560 [letter].

21. Okamura K, Inoue K, Omae T. A case of Hashimoto’s thyroiditis with thyroid immunological abnormality manifested after habitual ingestion of seaweed. Acta Endocrinol 1978;88:703–12.

22. Kim JY, Kim KR. Dietary iodine intake and urinary iodine excretion in patients with thyroid diseases. Yonsei Med J 41:22–8.

23. Walkiw O, Douglas DE. Health food supplements prepared from kelp--a source of elevated urinary arsenic. Can Med Assoc J 1974;111:1301–2 [letter].

24. Conz PA, La Greca G, Benedetti P, et al. Fucus vesiculosus: A nephrotoxic alga? Nephrol Dial Transplant 1998;13:526–7 [letter].

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